Theoretical studies on stable geometries of C_(78)O_6 based on C_(2v)-C_(78)

摘要

The semi-empirical AMI and INDO/CIS methods as well as density function theory have been used to study equilibrium geometries and spectroscopic properties of the possible isomers of C_(78)O_6 based on C_(2v)-C_(78). The most stable geometry at the ground state of C_(78)O_6 is found to be 28, 29, 30, 31, 52, 53, 70, 71, 73, 78, 75, 76-C_(78)O_6 (A). The C(70)-C(71) and C(75)-C(76) bonds are opened up to form annulene-like structures, whereas the other four bonds are not broken and show epoxy features in C_(78)O_6 (A). The oxygen atoms added tend to approach the sites vacated by the previous oxygen atoms since the conjugation system in C_(2v)-C_(78) is destroyed at these sites. Based on the B3LYP/6-31G optimized geometries, the main absorptions in the electronic spectra and the stretching vibrations of the C=C bonds in the IR spectra of C_(78)O_6 isomers are blue shifted as compared with those of C_(2v)-C_(78) calculated using the INDO/CIS and AM1 method, respectively. The chemical shifts of the carbon atoms in the epoxy and annulene-type structures of C_(78)O_6 isomers, calculated at B3LYP/6-31G level, move upfield.